The present investigation was undertaken in 12 normal subjects to determine the magnitude of technical and biologic variability of six previously validated Doppler indexes of diastolic function. Technical variability due to the reader was small for each of the six Doppler indexes. Variability due to the technician was larger than for the reader, and became sizeable when measurements obtained in a single subject were-compared; however, variability due to the technician was negligible when mean differences between groups of subjects were analyzed, and none of these differences achieved statistical significances. Day-to-day variability (i.e., biologic variability) was larger than technical variability (reader and technician) for the great majority of the comparisons, both in the individual (28 of 30 comparisons) and group data analyses (27 of 30 comparisons). The two Doppler indexes assessing late diastolic events (i.e., maximal late diastolic (atrial) flow-velocity, and the ratio between maximal early and late flow velocity) did show significant change on a day-to-day basis (p is less than 0.05). In conclusion, Doppler indexes that represent a measure of the relaxation and early filling phases of diastole showed good reproducibility for group data analysis. Hence, these Doppler indexes can be utilized to assess left ventricular diastolic function noninvasively in large groups of patients. However, extensively utilized indexes assessing the atrial contribution to ventricular filling, such as maximal late (atrial) diastolic flow-velocity and the ratio between maximal early and late flow-velocities, have a relatively large variability for both individual and group data analysis; therefore, conclusions based on these late diastolic indexes should be drawn with caution.